Regulatory Matters (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Regulatory Assets [Line Items]
Regulatory assets	$ 1,413.2	$ 1,612.4
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	829.7	770.7
Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	355.3	549.2
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	65.7	62.4
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.0	30.0
Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	25.0	34.9
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.1	40.2
Debt redemption costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	17.9	19.8
IPL's electric transmission cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	8.3	16.6
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	60.2	88.6
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,113.5	1,217.9
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	798.6	746.2
IPL [Member] | Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	174.2	279.3
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	36.7	38.6
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	30.0	30.0
IPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	21.0	30.3
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	5.9	16.3
IPL [Member] | Debt redemption costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	12.2	13.6
IPL [Member] | IPL's electric transmission cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	8.3	16.6
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.6	47.0
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	299.7	394.5
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	31.1	24.5
WPL [Member] | Pension and other postretirement benefits costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	181.1	269.9
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	29.0	23.8
WPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	0	0
WPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.0	4.6
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	15.2	23.9
WPL [Member] | Debt redemption costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	5.7	6.2
WPL [Member] | IPL's electric transmission cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	0	0
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 33.6	$ 41.6